1. Field of the Invention
The present invention relates to a process for forming tapered holes through dielectric layers for making electrical contacts in semiconductor integrated devices. The proces is particularly useful for fabricating integrated MISFET devices.
2. Description of the Prior Art
With the reduction of dimensions of integrated devices the formation of the necessary electrical contacts with specific surface areas of the semiconducting substrate through an insulating dielectric layer becomes more and more difficult. A severe technical problem relates to the so-called "step coverage", i.e. to the more or less satisfactory ability to ensure that the metal which is deposited to fill micrometric or sub-micrometric holes etched through a dielectric layer and conformally above the surface of the latter, has the necessary continuity and uniformity of thickness and is not prone to develop fractures in particularly critical points, such as on vertical steps and over sharp edges of the surface over which the metal is deposited. In order to avoid these phenomena it is expedient to taper the contact holes, i.e. incline the vertical steps of the front profile of the cross section of the wafer on which the metal must be deposited during the fabrication process. These practices often require high resolution masking techniques as well as special aggressive anisotropic etching techniques of the dielectric through a photoresist mask, such as RIE plasma etching, in order to ensure the necessary dimensional control during the formation of such sub-micrometric structural features as contact holes.
A commonly employed technique for smoothing the rim edge of etch holes through the dielectric contemplates a thermal "reflow" of the dielectric material itself. However this treatment has the disadvantage of being necessarily carried out during the terminal steps of the process of fabrication of integrated devices and may alter the diffusion profiles of the various doping species relative to regions of the monocrystalline semiconductor substrate already critically defined in devices with a high level of miniaturization. Moreover, especially in case of particularly minuscule contacts, the reflow heat treatment of the dielectric may indeed "choke" an etched hole.
Another solution lately proposed is based upon an optical tapering technique, which contemplates the formation of a "tapered image" in the photosensitive masking layer and a strict control of the etching selectivity of the masking photoresist material and of the dielectric material for producing an inclined profile of the hole being etched through the thickness of the dielectric layer. Notwithstanding the fact that this technique has the advantage of being substantially a "cold" operation and therefore without negative effects upon the already established diffusion profiles, it requires an intrinsically low contrast photolithographic process and this is detrimental to dimensional control.
A further proposal has been of carrying out after a first partial anisotropic etching step of the dielectric, a second etching step under isotropic conditions for completing the etching of the residual thickness on the bottom of the etch hole and simultaneously causing a tapering of the top portion of the hole pre-etched through the thickness of the dielectric material layer (round etch). Also this technique has problems relating to maintaining satisfactory dimensional control.